Earlier this week, Mozilla Security researcher Cody Crews discovered a malicious advertisement on a Russian news site that steals local files from a system and upload them to a Ukrainian server without the user ever knowing.

The malicious advertisement was exploiting a serious vulnerability in Firefox's PDF Viewer and the JavaScript context in order to inject a script capable of searching sensitive files on user's local file systems.

Mozilla versions of Firefox that do not contain the PDF Viewer, such as Firefox for Android, are not affected by the "Same origin violation and local file stealing via PDF reader" vulnerability.

The exploit does not execute any arbitrary code but injects a JavaScript payload into the local file context, allowing the script to search for and upload potentially user’s sensitive local files.

All an attacker need to do is load the page with this exploit and sit back and relax. The exploit will silently steal files in the background.

According to Mozilla lead security researcher Daniel Veditz, the exploit specifically searches for:

Any files encountered by the exploit are uploaded to a server in Ukraine.

"The exploit leaves no trace it has been run on the local machine," Veditz wrote in a blog post. "If you use Firefox on Windows or Linux it would be prudent to change any passwords and keys found in the above-mentioned files if you use the associated programs."

Mac users are currently safe from this exploit, but researcher warned that another payload could potentially exploit the same vulnerability to target Mac systems.

All versions of Firefox are affected, but the good news is that Mozilla has fixed the issue in its software. So, users are recommended to update browsers to Firefox 39.0.3 to protect against the exploit. Enterprise users can patch to 38.1.1.